1. Field of the Invention
The present invention relates to an objective lens used in an optical pickup that performs recording and/or reproducing of information signals on three different types of optical discs, the optical pickup, and an optical disc apparatus using the optical pickup.
2. Description of the Related Art
In recent years, there have been proposed, as a next-generation optical disc format, optical discs capable of high density recording, where signals are recorded and reproduced using a light beam of a blue-violet semiconductor laser beam having a wavelength around 405 nm (hereafter referred to as “high density recording optical disc”). High density recording optical discs are being proposed with a structure in which the cover layer for protecting the signal recording layer is thin, 0.1 mm for example.
When providing an optical pickup compatible with such high density recording optical discs, it is preferable that the optical pickup should be compatible with optical discs used in the related art. Specifically, it is preferable to have compatibility with CDs (Compact Discs) using a wavelength around 785 nm and DVDs (Digital Versatile Discs) using a wavelength around 655 nm, which have different formats, in the related art. That is to say, there is demand for an optical pickup and optical disc apparatus having compatibility among optical discs of multiple formats with different disc structures and accordingly different laser specifications.
In the related art, there has been a method of performing recording or reproducing of information signals on three types of optical discs of different formats by using two types of objective lenses. Specifically, there are optical pickups in which the two types of the objective lenses and two types of optical systems for DVD/CD and high density recording optical discs are provided and the respective objective lenses are switched in accordance with a wavelength used therein.
However, such optical pickups according to the related art have the following problems. First, since two objective lenses are used in the optical pickup, the attachment angle of the actuator of the objective lenses to the lens holders may be unsuitable. Thereby, it is difficult to set an optimal slope of the objective lens with respect to an optical disc, and this causes a problem of deterioration in quality of reproduced signals. Second, in such optical pickups, the number of components which should be placed along the optical path of each of the two optical systems increases. Accordingly, there are problems of an increase in cost and an increase in the size of the optical pickup. Moreover, in the above-mentioned optical pickups, it is necessary for the two objective lenses to be mounted on the actuator for driving the objective lenses. Hence, the weight of the actuator increases, and this causes a problem of deterioration in sensitivity.
As opposed to the above-mentioned configuration, in order to solve the above-mentioned problems and further simplify optical components, there has been studied an optical pickup that has a single common objective lens for a plurality of types of optical discs and three used wavelengths. A basic principle for providing an objective lens for three-wavelength light beams is to provide a diffractive portion in the optical path and thereby diffract the light beams in a divergent direction or in a convergent direction, thereby correcting spherical aberration caused by the combination of the used wavelengths and the media.
However, in the optical pickup, which has been studied in the related art, capable of achieving three-wavelength compatibility by using a single objective lens, it has not been considered that unwanted diffracted light caused by the diffraction structure is increased by change in temperature and wavelength. That is, such unwanted light may form an image on the light sensing element similarly to regular light. Hence, in particular, at the time of reading a double layer optical disc, a signal generated from another layer interferes with a focus servo signal, and thus a problem arises in that it is difficult to perform a stable focus servo.
Japanese Unexamined Patent Application Publication No. 2004-265573 is an example of the related art.